1. Technical Field of the Invention
The present invention relates generally to power generation controls for electric generators. More particularly, the invention relates to a control apparatus for an electric generator, which can economically and reliably control the power generating operation of the electric generator.
2. Description of the Related Art
In conventional electrical systems for automobiles, a control apparatus (or a voltage regulator) generally controls the power generating operation of an automotive alternator so as to keep the voltage of a power supply bus within a predetermined range.
More specifically, the control apparatus determines a change in the voltage of the power supply bus and controls the power generating operation of the alternator according to the change. The change is caused by a difference between the amount of electric power required by electrical loads connected to the power supply bus and that supplied by electric power sources including the alternator and an automotive battery connected to the power supply bus. The amount of electric power supplied by the electric power sources is equal to the sum of the amount of electric power generated by the alternator and that discharged from (i.e., +) or charged into (i.e., −) the battery. In other words, the control apparatus controls the power generating operation of the alternator to regulate the terminal voltage of the battery to a reference voltage. Accordingly, such a way of power generation control is referred to as constant voltage control.
With the constant voltage control, however, the control apparatus performs a control process on the alternator only after the change in the voltage of the power supply bus has taken place. Consequently, it is difficult to achieve a high control response and to secure high stability of the power supply bus.
To solve the above problem, Japanese Patent First Publications No. 2004-249900 and 2004-194495 disclose another way of power generation control, called power balance control.
According to the power balance control, the control apparatus controls the power generating operation of the alternator to keep balance between the amount of electric power required by the electrical loads and that supplied by the electric power sources, thereby keeping the voltage of the power supply bus within the predetermined range.
With the power balance control, the control apparatus performs a control process on the alternator for keeping balance between the amount of electric power required by the electrical loads and that supplied by the electric power sources when or before an unbalance therebetween occurs. Consequently, a high control response and high stability of the power supply bus can be expected.
Japanese Patent First Publications No. 2002-118905 and 2004-260908 disclose yet another way of power generation control, called power generation cost control.
According to the power generation cost control of Japanese Patent First Publication No. 2004-260908, the control apparatus first determines the power generation cost of each of a plurality of electric power sources in the electrical system. Then, based on the determined power generation costs of the electric power sources, the control apparatus adjusts the power supply percentages of the electric power sources so as to minimize the total power generation cost of the electrical system.
More specifically, the power generation cost of the alternator is determined as an increase in fuel consumption of the engine for driving the alternator to generate unit electric power. The power generation cost of the battery is, on the other hand, determined as the cost for charging unit electric power into the battery. To minimize the total power generation cost of the electrical system, the control apparatus controls the alternator to generate electric power to charge the battery when the power generation cost of the alternator is lower than that of the battery, and controls the battery to discharge when the power generation cost of the battery is lower than that of the alternator. Consequently, a notable improvement in fuel economy of the automobile can be expected.
However, to secure reliability of the power generation cost control, it is required to take measures for keeping the battery from being over-charged or over-discharging. Further, it is also required to take into consideration the fact that the electric energy storage capability and discharge capability of the battery gradually change with repeated charge and discharge operations. Consequently, the control apparatus comes to perform complicated computation so as to meet the above requirements.